An intelligent optical network (for example, optical communication network) mainly consists of a series of equipment with high flexibility, where a Reconfigurable Optical Add/Drop Multiplexer (ROADM) is a core functional module in the intelligent optical network. A conventional ROADM adopts a 1×M WSS (M>1), and achieves connection among network nodes by software configuration and automatic power balancing to achieve adding, dropping and direct connection of a wavelength. However, because the conventional ROADM based on the 1×M WSS has a limited number of ports, the number of ROADMs in the optical network increases sharply with development of the intelligent optical network into a higher dimension, network management becomes more complicated, and power consumption cost increases accordingly. Therefore, increasing the number of input ports and output ports of a single ROADM becomes one of the key issues to be solved for a next generation of ROADM technology. To this end, an N×M WSS (N>1, M≧1) which can be used in the ROADM is proposed. However, a conventional N×M WSS cannot implement that optical signals of different wavelengths input from different input ports are output from a same output port.
Therefore, a new wavelength selective switch and wavelength selection method are needed, which not only can implement that an optical signal of any wavelength input via any input port is output via any output port, but also can implement that optical signals of different wavelengths input via different input ports are output via a same output port.